1. Field of the Invention
The present invention relates to an inverter, and more particularly to an inverter in which its output voltage level is adjusted on a real time basis in compliance with an output current level of the inverter, thereby continuously driving an induction motor at its efficiency optimal operating point.
2. Description of the Prior Art
Most of the prior art inverters employ a voltage/frequency (v/f) pattern as a predetermined set so that a command voltage is determined when a command frequency is selected. Namely, as indicated by FIG. 1, a command frequency and command voltage set is either a constant torque characteristic indicated by a straight line 01, reduced torque characteristic shown by a curved line 02, or a constant power v/f pattern illustrated by a straight line 03. A desired characteristic or a combination thereof can be selected manually among these characteristics.
When load characteristics are known for a prior art inverter, the most suitable characteristic can be selected for a given load. However, when an induction motor is employed as an actuator, desired motor speed and torque vary as functions of time. Even when an induction motor is utilized as a quasi-steady power generator, it is often the case that its load is either unknown or varying. Under these circumstances, the induction motor is being driven at an off-optimal operating point, the input electric energy cannot be converted efficiently to an output mechanical energy.
When an alternating current with a constant voltage is applied to a stator winding, this becomes an excitation current for an induction motor, generating a rotating magnetic field. An eddy current, i.e. load current is then induced on the motor rotor. An interaction between the rotating magnetic field and the magnetic field generated by the eddy current on the rotor will then generate torque to drive the load coupled to the rotor.
If the magnitude relationship between the voltage and current applied to the induction motor is unmatched, motor efficiency will be degraded. Namely, if the magnitude of the applied voltage is too large compared to the level of the load, iron loss (hysteresis loss of the magnetic circuit) and copper loss (resistance loss of the winding) will increase. These iron and copper losses will increase their magnitude almost proportionately to the circuit current squared. If, on the other hand, the magnitude of the applied voltage is too small compared to the level of the load, excitation current through the winding becomes insufficient to generate a necessary rotating magnetic field. To compensate for this insufficiency, the load current will increase drastically resulting in a secondary copper loss, which is almost proportinal to the load current squared.